TLS N. Cam-Winget
Internet-Draft Cisco Systems
Intended status: Informational J. Visoky
Expires: December 10, 2020 ODVA
June 8, 2020

TLS 1.3 Authentication and Integrity only Cipher Suites


There are use cases, specifically in Internet of Things (IoT) and constrained environments that do not require confidentiality, though mutual authentication during tunnel establishment and message integrity is still mandated. This document defines the use of HMAC only cipher suites for TLS 1.3.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on December 10, 2020.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

There are several use cases in which communications privacy is not strictly needed, although authenticity of the communications transport is still very important. For example, within the Industrial Automation space there could be TCP or UDP communications which command a robotic arm to move a certain distance at a certain speed. Without authenticity guarantees an attacker could modify the packets to change the movement of the robotic arm, potentially causing physical damage. However, the motion control commands are not considered to be sensitive information and thus there is no requirement to provide confidentiality. Another IoT example with no strong requirement for confidentiality is the reporting of weather information; however, message authenticity is required to ensure integrity of the message..

Besides having a strong need for authenticity and a weak need for confidentiality, many of these systems also have serious latency requirements. Furthermore, several IoT devices (industrial or otherwise) have limited processing capability. However, these IoT systems still gain great benefit from leveraging TLS 1.3 for secure communications. Given the reduced need for confidentiality TLS 1.3 [RFC8446] cipher suites that maintain data integrity without confidentiality are described in this document.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Applicability Statement

The cipher suites defined in this document are intended for a small limited set of applications where confidentiality requirements are relaxed and the need to minimize the cryptographic algorithms are prioritized. This section describes some of those applicable use cases.

Use cases in the industrial automation industry, while requiring data integrity, relax the confidential communications requirement. Mainly, information communicated to unmanned machines to execute repetitive tasks do not convey private information. For example, there could be a system with a robotic arm that is doing high speed pick-and-place of materials. The position synchronization data and motion commands are required to have very low latency, as the process needs to be done at high speed on a compute and memory constrained device. However, information such as the position, speed, acceleration of the robotic arm or other material in the system is not confidential. That is, while an attacker can determine the behavioral aspects and task of the device; no intellectual property concerns or data privacy concerns exist for these communications. However, data integrity is required as being able to modify this data would be a threat that an attacker might seek to exploit with serious consequences; the attacker could modify the motion information in order to cause physical damage to the equipment.

Another use case which is closely related is that of fine grained time updates. Motion systems often rely on time synchronization to ensure proper execution. Time updates are essentially public, there is no threat from an attacker knowing the time update information. This should make intuitive sense to those not familiar with these applications; rarely if ever does time information present a serious attack surface dealing with privacy. However the authenticity is still quite important. Modification of the data can at best lead to a denial-of-service attack, although a more intelligent threat actor might be able to cause actual physical damage. As these time synchronization updates are very fine-grained, it is again important for latency to be very low.

A third use case deals with Alarming data. Industrial control sensing equipment can be configured to send alarm information when it meets certain conditions. Often times this data is used to detect certain out-of-tolerance conditions, allowing an operator or automated system to take corrective action. Once again, in many systems the reading of this data doesn't grant the attacker information that can be exploited, it is generally just information regarding the physical state of the system. At the same time, being able to modify this data would allow an attacker to either trigger alarms falsely or to cover up evidence of an attack that might allow for detection of their malicious activity. Furthermore, sensors are often low powered devices that might struggle to process encrypted and authenticated data. Sending data that is just authenticated significantly eases the burden placed on these devices, yet still allows the data to be protected against any tampering threats.

A fourth use case considers the protection of commands in the railway industry. In railway control systems, no confidentiality requirements are applied for the command exchange between an interlocking controller and a railway equipment controller (for instance, a railway point controller of a tram track where the position of the controlled point is publicly available). However, protecting integrity of those commands is vital, otherwise, an adversary could change the target position of the point by modifying the commands, which consequently could lead to the derailment of a passing train. Furthermore, requirements for providing blackbox recording of the safety related network traffic can only be fulfilled through using integrity only ciphers, to be able to provide the safety related commands to a third party, which is responsible for the analysis after an accident.

The above use cases describe the relaxed requirements to provide confidentiality, and as these devices come with a small runtime memory footprint and reduced processing power, the need to minimize the number of cryptographic algorithms used is prioritized.

4. Cryptographic Negotiation Using Integrity only Cipher Suites

The cryptographic negotiation as specified in [RFC8446] Section 4.1.1 remains the same, with the inclusion of the following cipher suites:

{0xC0, 0xB4}
{0xC0, 0xB5}

These cipher suites allow the use of SHA-256 or SHA-384 as the HMACs for data integrity protection as well as its use for HKDF. The authentication mechanisms remain unchanged with the intent to only update the cipher suites to relax the need for confidentiality.

Given that these cipher suites do not support confidentiality, they MUST only be used with certificate-based authentication and Diffie-Hellman key exchange.

5. Record Payload Protection for Integrity only Cipher Suites

Given that there is no encryption to be done at the record layer, the operations "Protect" and "Unprotect" take the place of "AEAD-Encrypt" and "AEAD-Decrypt", respectively.

The record payload protection as defined in [RFC8446] can be retained when integrity only cipher suites are used. This section describes the mapping of record payload structures when integrity only cipher suites are employed.

As integrity is provided with protection over the full record, the encrypted_record in the TLSCiphertext along with the additional_data input to protected_data (termed AEADEncrypted data in [RFC8446]) as defined in Section 5.2 [RFC8446] remains the same. The TLSCiphertext.length for the integrity cipher suites will be:

TLSCiphertext.length = TLSPlaintext.length + 1 (type field) + length_of_padding + 32 (HMAC) = TLSInnerPlaintext_length + 32 (HMAC)
TLSCiphertext.length = TLSPlaintext.length + 1 (type field) + length_of_padding + 48 (HMAC) = TLSInnerPlaintext_length + 48 (HMAC)

Note that TLSInnerPlaintext_length is not defined as an explicit field in [RFC8446], this refers to the length of the TLSInnterPlaintext structure

The resulting protected_record is the concatenation of the TLSInnerPlaintext with the resulting HMAC. With this mapping, the record validation order as defined in Section 5.2 of [RFC8446] remains the same. That is, encrypted_record field of TLSCiphertext is set to:

TLSCiphertext = TLS13-HMAC-Protected = TLSInnerPlaintext || HMAC(write_key, nonce || additional_data || TLSInnerPlaintext)

Here "nonce" refers to the per-record nonce described in section 5.3 of [RFC8446].

The Protect and Unprotect operations provide the integrity protection using HMAC SHA-256 or SHA-384 as described in [RFC6234].

Due to the lack of encryption of the plaintext, record padding is not needed, although it can be optionally included.

6. Key Schedule when using Integrity only Cipher Suites

The key derivation process for Integrity only Cipher Suites remains the same as defined in [RFC8446]. The only difference is that the keys used to protect the tunnel applies to the negotiated HMAC SHA-256 or HMAC SHA-384 ciphers. Note that the traffic key material (client_write_key, client_write_iv, server_write_key and server_write_iv) MUST be calculated as per RFC 8446, section 7.3. The key lengths and IVs for these cipher suites are according to the hash lenghts. In other words, the following key lenghts and IV lengths SHALL be:

Cipher Suite Key Length IV Length
TLS_SHA256_SHA256 32 Bytes 32 Bytes
TLS_SHA384_SHA384 48 Bytes 48 Bytes

7. Error Alerts

The error alerts as defined by [RFC8446] remains the same, in particular:

bad_record_mac: This alert can also occur for a record whose message authentication code can not be validated. Since these cipher suites do not involve record encryption this alert will only occur when the HMAC fails to verify.

decrypt_error: This alert as described in [RFC8446] Section 6.2 occurs when the signature or message authentication code can not be validated.

8. IANA Considerations

IANA has granted registration the following specifically for this document:

TLS_SHA256_SHA256 {0xC0, 0xB4} cipher suite and TLS_SHA384_SHA384 {0xC0, 0xB5} cipher suite.

Note that both of these cipher suites are registered with the DTLS-OK column set to Y and the Recommneded column set to N

9. Security and Privacy Considerations

In general, with the exception of confidentiality and privacy, the security considerations detailed in [RFC8446] and in [RFC5246] apply to this document. Furthermore, as the cipher suites described in this document do not provide any confidentiality, it is important that they only be used in cases where there are no confidentiality or privacy requirements and concerns; and the runtime memory requirements can accommodate support for more cryptographic constructs.

With the lack of data encryption specified in this draft, no confidentiality or privacy is provided for the data transported via the TLS session. To highlight the loss of privacy, the information carried in the TLS handshake, which includes both the Server and Client certificates, while integrity protected, will be sent unencrypted. Similarly, other TLS extensions that may be carried in the Server's EncryptedExtensions message will only be integrity protected without provisions for confidentiality. Furthermore, with this lack of confidentiality, PSK data MUST NOT be sent in the handshake while using these cipher suites.

Given the lack of confidentiality, it is of the utmost importance that these cipher suites never be enabled by default. As these cipher suites are meant to serve the IoT market, it is important that any IoT endpoint that uses them be explicitly configured with a policy of non-confidential communications.

10. Acknowledgements

The authors would like to acknowledge the work done by Industrial Communications Standards Groups (such as ODVA) as the motivation for this document. We would also like to thank Steffen Fries for providing a fourth use case. In addition, we are grateful for the advice and feedback from Joe Salowey, Blake Anderson, David McGrew, Clement Zeller, and Peter Wu.

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018.

11.2. Informative Reference

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.

Authors' Addresses

Nancy Cam-Winget Cisco Systems 3550 Cisco Way San Jose, CA 95134 USA EMail:
Jack Visoky ODVA 1 Allen Bradley Dr Mayfield Heights, OH 44124 USA EMail: